1. Field of the Invention
The present invention relates to a high dielectric ceramic composition having a highly balanced dielectric constant and Q value which imparts a high breakdown voltage and thus expands the range of operation comditions, insures stable operation, and imparts special effects such as dielectric temperature compensation wherein the temperature coefficients of usual electrical circuit elements can be compensated by the high dielectric constant. Accordingly, the invention relates to an optimally high dielectric ceramic composition.
2. Description of the Prior Art
Known ceramic compositions with dielectric constants of 10- 500 and temperature coeffieients of +100.about.- 5000 .times. 10.sup.-.sub.6 /.degree.C include BaTiO.sub.3, SrTiO.sub.3, CaTiO.sub.3, MgTiO.sub.3, or La.sub.2 O.sub.3. TiO.sub.2, MgO. SiO.sub.2, Bi.sub.2 O.sub.3.2TiO.sub.2, and the like. However, compositions with a balanced combination of high dielectric constant, low temperature coefficient, and high Q value have been difficult to obtain. In such compositions, the temperature coefficient increases with increasing dielectric constant. Therefore, when a low temperature coefficient is required, a decreased dielectric constant must be tolerated along with a deterioration of loss angle at high frequency. Accordingly, even if the temperature coefficient of conventional compositions for temperature compensation can be decreased, deteriorated dielectric constant and Q value can be expected in electronic circuit elements, whereby sufficient temperature compensation will not result.
A high dielectric ceramic composition for temperature compensation which has a low and controlled temperature coefficient, high electric constant and low loss angle at high frequency is the CaO--TiO.sub.2 -- SiO.sub.2 --SrO system. However, the dielectric constant of this composition is only 18.4- 145 (MHz at 25.degree.C), as compared with the dielectric constant of CaTiO.sub.3 --Sb.sub.2 O.sub.3.2MgO--SrO.Nb.sub.2 O.sub.3 system which is 64- 144. A practical temperature compensation requires a high dielectric constant. In order to improve the safety range for the conditions of operation, an increased breakdown voltage is also essential. However, no ceramic which can impart 20 KV/mm of AC breakdown voltage per 1 mm is known. The CaTiO.sub.3 --Sb.sub.2 O.sub.3. 2MgO--SrO.Nb.sub.2 O.sub.3 system imparts a maximum AC breakdown voltage of only 10 KV /mm, which is similar to values for conventional compositions.
Temperature coefficients of various electronic circuit elements for temperature compensation are positive and relatively high. Accordingly, a practical temperature compensation effect cannot be expected with a ceramic composition for temperature compensation which has a negative and low temperature coefficient. For example, ferrite is usually used as the magnetic core for an I.F.T. circuit which is indispensable in a color T.V. The temperature coefficient of ferrite is in the range of +500.about.+2000 .times. 10.sup.-.sub.6 .degree.C. Accordingly, a negative temperature coefficient is necessary for temperature compensation of practical circuit elements and also to give balanced high characteristics. In a resonance circuit having ##EQU1## wherein L designates a positive temperature coefficient, sufficient temperature compensation C must be imparted to obtain excellent reliability.
A need exists therefore, for a composition which will increase breakdown voltage while maintaining high dielectric constant and temperature compensation effects.